1. Field of Invention
The invention relates to an automatic transmission provided in vehicles, and more particularly, to a hydraulic control system for controlling engaging elements in the transmission mechanism.
2. Description of Related Art
In the systems in which electrically controlled solenoid valves supply a hydraulic pressure to respective hydraulic servos of a plurality of engaging elements, the hydraulic pressure may be simultaneously supplied from a plurality of solenoid valves in the event of the electrical failure or sticking of the valves (hereinafter, collectively referred to as xe2x80x9cfailurexe2x80x9d). In such a case, three or more of the engaging elements may be simultaneously engaged, whereby the transmission may be interlocked. In order to prevent such a problem, a valve for supplying the hydraulic pressure only to two hydraulic servos is provided in the system. Such conventional art is disclosed, for example, in U.S. Pat. No. 4,903,551 or U.S. Pat. No. 5,010,786.
The conventional art is structured to ensure that, in the case of the failure, the hydraulic pressure is supplied to the hydraulic servos such that two engaging elements attaining the third speed are engaged. The third speed is assured because, at the third speed, a certain driving force required for limphome control in the forward four-speed gear train of the conventional art can be obtained, while preventing an abrupt engine brake resulting from shifting to the lower gear even upon failure at the highest, fourth speed.
In the case where the higher speed (e.g., the third speed in the above example) is ensured in the event of the failure, the vehicle may possibly fail to climb a steep hill during the limphome control, and/or may fail to run over a slight difference in level of the road during running at the low speed. However, if a lower speed is assured upon failure, an abrupt engine brake may occur. In view of the above, it is desirable that the running state be changed before and after the recognition of the failure by the driver, and that the speed to be attained is varied according to the driving force required to change the running state. In spite of such requirements, it is practically difficult to shift the gear during the failure.
The invention is made in view of the foregoing, and it is an object of the invention to provide a hydraulic control system of an automatic transmission, which is capable of attaining the higher speed during running at a high speed, while also attaining another speed to assure the driving force, in the event of the failure. Preferably, the automatic transmission has brakes, clutches and/or one-way-clutches as engaging elements.
In order to achieve the above-mentioned object, a hydraulic control system of an automatic transmission includes a plurality of engaging elements, a plurality of hydraulic servos each receiving a hydraulic pressure so as to control the respective engaging elements, and a plurality of shifting means for outputting the hydraulic pressure supplied to the respective hydraulic servos to achieve a plurality of forward speeds by engaging and disengaging the corresponding engaging elements. The hydraulic control system includes switching means for switching to a predetermined state where a specific speed among the plural speeds is achieved by cutting off a hydraulic pressure supply path to a specific hydraulic servo upon receipt of the output hydraulic pressure from all of said shifting means ready for outputting hydraulic pressure; and supply switching means, disposed in a hydraulic pressure supply path to a first hydraulic servo that is not included in the specific servo, for switching between supply and cut-off states of the hydraulic pressure to the first hydraulic servo.
More specifically, it is advantageous that the switching means receives as a signal pressure at least the hydraulic pressure supplied to the first hydraulic servo and is switched to the predetermined state only when the signal pressure is applied so as to achieve the specific speed by cutting off the supply of the hydraulic pressure to the specific hydraulic servo, and when the signal pressure is not applied so as to achieve another speed by continuing supply of the hydraulic pressure to a second hydraulic servo included in the specific hydraulic servo, the supply switching means allows the signal pressure to be supplied in a supply state and does not allow the signal to be supplied in the cut-off state.
Furthermore, it is advantageous that the hydraulic control system of an automatic transmission further includes a solenoid valve that applies a signal pressure for switching the supply switching means, and the solenoid valve switches the supply switching means to the supply side by applying the signal pressure to the supply switching means when achieving a speed with which the first hydraulic servo is engaged.
Moreover, it is advantageous that the solenoid valve applies the signal pressure to the supply switching means so as to be brought into a cut-off state in a de-energized state, and the supply switching means receives hydraulic pressure of the first hydraulic servo and is kept at the supply side by the hydraulic pressure supplied to the first hydraulic servo.
Moreover, in order to apply the invention to a multi-speed transmission, it is advantageous that the specific hydraulic servo includes a third hydraulic servo; the hydraulic servo which is not included in the specific hydraulic servo includes a fourth hydraulic servo; and the shifting means for supplying the respective hydraulic pressures to the third and fourth hydraulic servos is located downstream of the switching means, and a shifting valve for selectively shifting between the third and fourth hydraulic servos is disposed.
In order to avoid complication of the circuit structure, it is advantageous that the shifting valve receives the hydraulic pressure of the first hydraulic servo, and is switched in accordance with a state of the hydraulic pressure supplied to the first hydraulic servo.
Moreover, in the case where the invention is applied to a four-speed transmission, it is advantageous that a plurality of hydraulic servos include a hydraulic servo of a first clutch engaged upon attaining forward first to third speeds; a hydraulic servo of a second clutch engaged upon attaining the third and fourth speeds; a hydraulic servo of a first brake engaged upon attaining the second and fourth speeds; and a hydraulic servo of a second brake engaged upon attaining the first speed. In the case where the hydraulic pressure is output from the respective shifting means to all of the four hydraulic servos, the switching means includes a first switching valve that cuts off supply of hydraulic pressure to the hydraulic servo of the second brake by the hydraulic pressure supplied to the hydraulic servo of the first brake when the hydraulic servo of the second brake is in a state where the hydraulic pressure can be supplied, and further cuts off supply of the hydraulic pressure to the hydraulic servo of the second brake by the hydraulic pressure when the hydraulic servo of the second clutch is in the state where the hydraulic pressure can be supplied, and a second switching valve that cuts off supply of the hydraulic pressure to the first clutch by supplying the hydraulic pressure to the hydraulic servo of the first brake.
Moreover, in the case where the invention is applied to a six-speed transmission, it is preferred that a plurality of hydraulic servos include a hydraulic servo of a first clutch engaged upon attaining forward first to fourth speeds; a hydraulic servo of a second clutch engaged upon attaining the fourth to sixth speeds; a hydraulic servo of a third clutch engaged upon attaining the third and fifth speeds; a hydraulic servo of a first brake engaged upon attaining the second and sixth speeds; and a hydraulic servo of a second brake engaged upon attaining the first speed. In the case where the hydraulic pressure is output from the respective shifting means to all of the five hydraulic servos, the switching means includes first and second switching valves each cutting off supply of hydraulic pressure to the hydraulic servo of the first clutch from the corresponding shifting means by hydraulic pressure supplied to the hydraulic servo of the second clutch, a third switching valve selectively communicating the shifting means of the hydraulic servo of the third clutch and the shifting means of the hydraulic servo of the first brake with the hydraulic servo of the third clutch, and a fourth switching valve for cutting off supply of the hydraulic pressure to the hydraulic servo of the second brake by the hydraulic pressure which is selectively supplied to the respective hydraulic servos of the third clutch and the first brake.
According to one aspect of the invention, if the supply switching means is supplying the hydraulic pressure to the first hydraulic servo in the event of the failure which causes all the shifting means to output a hydraulic pressure, the specific speed is attained by the switching means. On the other hand, if the supply switching means is not supplying the hydraulic pressure to the first hydraulic servo in the event of the failure, the switching means is not switched to the prescribed state, and therefore, does not cut off the hydraulic pressure to the specific hydraulic servo. As a result, a speed other than the specific speed is attained. Accordingly, in the event of the failure, at least two speeds can be achieved depending upon the state of the supply switching means.
According to another aspect of the invention, due to the cooperative motion of the switching means and the supply switching means, at least two speeds can be achieved in the event of the failure, depending upon the state of the supply switching means.
According to another aspect of the invention, in the event of the failure, the supply switching means is switched to the supply side by the solenoid valve. Accordingly, the speed can be successfully achieved in the normal condition.
According to another aspect of the invention, the solenoid valve and the shifting means according to the invention are de-energized as a result of the failure. In the case where the hydraulic pressure is being supplied to the first hydraulic servo at the time of the failure, the supply switching means has been switched to the supply side by the hydraulic pressure of the first hydraulic servo. Accordingly, a specific speed is achieved by the switching means even if the solenoid valve is de-energized. On the other hand, in the case where the hydraulic pressure is not being supplied to the first hydraulic servo at the time of the failure, the supply switching means is switched to the cut-off side since the solenoid valve is de-energized. Accordingly, another speed is achieved by the switching means. Thus, two speeds can be achieved even if the solenoid valve also fails electrically. Moreover, due to the engine stop or the setting of the neutral range, the supply of the hydraulic pressure to the first hydraulic servo can be cut off at the time of the failure. In this case, the supply switching means is switched to the cut-off side, whereby another speed is achieved. Accordingly, setting the specific speed as a high speed and another speed as a low speed may prevent an abrupt engine braking during running as well as assure the driving force.
According to another aspect of the invention, instead of cutting off supply of the hydraulic pressure to the second hydraulic servo of the specific hydraulic servos to be cut off, the supply of the hydraulic pressure to the first hydraulic servo is cut off by the supply switching means, whereby another speed can be attained. In this case, however, the another speed is limited to the speed achieved by cutting off the supply of the hydraulic pressure to the third hydraulic servo. Therefore, by cutting off the fourth hydraulic servo that is not included in the specific servos and is different from the first hydraulic servo, instead of cutting off the third hydraulic servo, a choice of attainable speeds is increased. Accordingly, by shifting the connection between the shifting means and the hydraulic servos in the downstream of the switching means, supply of the hydraulic pressure to the fourth hydraulic servo is cut off instead of the third hydraulic servo. Therefore, the number of attainable speeds is increased.
According to another aspect of the invention, the shifting valve is switched based upon a state of the hydraulic-pressure supply to the first hydraulic servo. Therefore, the shifting valve can be switched in synchronization with the switching of the supply switching means. Accordingly, a special solenoid valve for the synchronized operation or the like is not required.
According to another aspect of the invention, the second speed is attained in the event of the failure at the first and second speeds, and the fourth speed is attained in the event of the failure at the third and fourth speeds. Therefore, even if the failure occurs during running of the vehicle, the gear is not shifted to the speed less than that attained during running. As a result, an abrupt engine brake can be prevented. Moreover, the second brake can be attained which provides driving force large enough to start the vehicle. Therefore, the minimum driving force required for running can be assured.
According to another aspect of the invention, the third speed is attained in the event of the failure at the first to third speeds, and the sixth speed is attained in the event of the failure at the fourth to sixth speeds. Therefore, even if the failure occurs during running of the vehicle, the gear is not shifted to the speed less than that attained during running. As a result, an abrupt engine brake can be prevented. Moreover, the third brake can be attained which provides great driving force sufficient to start the vehicle. Therefore, the minimum driving force required for running can be assured.